The new movie Hustlers is based on a true story of a group of strippers in New York City who fleece dozens of wealthy men during the late-2000s financial crisis. In the film, which hits theaters September 13, Jennifer Lopez stars as Ramona Vega, a veteran stripper who serves as a mentor to newcomer Destiny (Constance Wu). To make her role as a top-tier stripper convincing, Lopez took a crash course with pole choreographer Johanna Sapakie—and, according to Lopez, “It might be one of the hardest” things she’s ever done for a movie.

Lopez shares her journey from practicing on the pole at home to shooting a key dance scene in a video she just uploaded to her YouTube channel.

“Cardi was telling me me, she was like it took her years,” Lopez says in the clip about her Hustlers costar Cardi B, who was a stripper before her music career took off. “She’s great at it, but it took her years to figure out all this stuff. And Johanna is trying to teach me in a matter of, like, six weeks.”

“This is just as hard as anything I’ve ever learned,” Lopez adds. “I’ve gotten cuts and bruises and stuff from movies, but I’ve never been bruised like this from anything I’ve done.”